Hematopoietic stem cells are multipotent stem cells having self-renewal potential and capable of differentiating into all types of hematopoietic cells such as leukocytes, erythrocytes, and platelets. They are present in bone marrow fluid, peripheral blood, and cord blood. It is known that hematopoietic stem cell transplant, which is the transplant of hematopoietic stem cells into a body, is an effective way of treating intractable blood disease such as leukemia. Although hematopoietic stem cells in bone marrow fluid or peripheral blood generally are used for the treatment, there is a problem in that their collection places a considerable burden on donors. In contrast, cord blood is obtained secondarily at the time of delivery, so that it places a lower burden on donors. Besides, cord blood is excellent in transplant compatibility. Thus, cord blood is attracting attention as a supply source of hematopoietic stem cells.
However, the amount of cord blood obtained from a donor is smaller than the amount of bone marrow fluid or peripheral blood, so that the cord blood may fail to provide a sufficient number of cells necessary for transplant. As a remedy therefor, there has been developed a method for proliferating cord blood-derived hematopoietic stem cells by culturing so that they can be used for transplant. As a specific example thereof, a method for accelerating the proliferation of the hematopoietic stem cells by culturing them together with mesenchymal stem cells, feeder cells derived from an animal of a different species, or the like has been reported (Patent Document 1 and Patent Document 2). When culturing the hematopoietic stem cells, it is desired to inhibit the proliferation and differentiation until a desired time, for example. Thus, for example, a method in which a virus vector incorporating a differentiation-inhibiting gene is transfected into the hematopoietic stem cells has been reported (Patent Document 3). However, regarding the method including the transfection of the virus vector, there is a fear that it may affect the expression of other genes, so that its safety might be insufficient, for example. It also has a problem of cumbersome operations and high cost.